The subject matter described herein relates generally to an energy exchange system for conditioning air in an enclosed structure, and more particularly, to a liquid-to-air membrane energy exchanger (LAMEE).
Enclosed structures, such as occupied buildings, factories and animal barns, generally include an HVAC system for conditioning ventilated and/or recirculated air in the structure. The HVAC system includes a supply air flow path and an exhaust air flow path. The supply air flow path receives pre-conditioned air, for example outside air or outside air mixed with re-circulated air, and channels and distributes the air into the enclosed structure. The pre-conditioned air is conditioned by the HVAC system to provide a desired temperature and humidity of supply air discharged into the enclosed structure. The exhaust air flow path discharges air back to the environment outside the structure. Without energy recovery, conditioning the supply air typically requires a significant amount of auxiliary energy. This is especially true in environments having extreme outside air conditions that are much different than the required supply air temperature and humidity. Accordingly, energy exchange or recovery systems are typically used to recover energy from the exhaust air flow path. Energy recovered from air in the exhaust flow path is utilized to reduce the energy required to condition the supply air.
Conventional energy exchange systems may utilize energy recovery devices (e.g. energy wheels and permeable plate exchangers) or heat exchange devices (e.g. heat wheels, plate exchangers, heat-pipe exchangers and run-around heat exchangers) positioned in both the supply air flow path and the return air flow path. LAMEEs are fluidly coupled so that a desiccant liquid flows between the LAMEEs in a run-around loop, similar to run-around heat exchangers that typically use aqueous glycol as a coupling fluid. When the only auxiliary energy used for such a loop is for desiccant liquid circulation pumps and external air-flow fans, the run-around system is referred to as a passive run-around membrane energy exchange (RAMEE) system, otherwise it is an active RAMEE system with controlled auxiliary heat and/or water inputs or extractions.
For the passive RAMEE system with one or more LAMEEs in each of the exhaust and supply air ducts, energy in the form of heat and water vapor is transferred between the LAMEEs in the supply and exhaust ducts, which is interpreted as the transfer of sensible and latent energy between the exhaust air and the supply air. For example, the exhaust air LAMEE may recover heat and moisture from the exhaust air to transfer the heat and moisture to the supply air during winter conditions to heat and humidify the supply air. Conversely, during summer conditions, the supply air LAMEE may transfer heat and moisture from the supply air to the exhaust air to cool and dehumidify the supply air.
Laboratory prototype LAMEEs have been constructed and tested in passive RAMEE loops to utilize both cross-flow and counter-flow arrangements for each LAMEE. In a counter-flow configuration, the desiccant liquid flows in a direction 180° away from the air flow direction in the adjacent air flow channel (i.e. counter-flow with respect to the air flow direction for each pair of flow channels) and heat and water vapor are transferred through the semi-permeable, energy exchange, membrane of each LAMEE. In the cross-flow arrangement, the liquid desiccant in the LAMEE flows at 90° or perpendicular to the air flow direction through each pair of channels in the LAMEE energy exchange membrane area.
Both counter-flow and cross-flow LAMEE devices can be used to recover energy from exhaust air-flows. This energy can be used to condition the supply air using another LAMEE device. Cross-flow LAMEEs are not without disadvantages. In certain circumstances, cross-flow exchangers generally have lower energy transfer effectiveness in comparison to counter-flow exchangers of the same energy exchange membrane area and inlet operating conditions. Accordingly, it may be desirable to have an energy exchange system that utilizes counter-flow LAMEEs. However, counter-flow LAMEEs are generally more difficult and expensive to construct. In particular, counter-flow LAMEEs require headers positioned on each end of the LAMEE and require tighter design specifications. Accordingly, conventional counter-flow LAMEEs may be impractical for some applications but, where higher performance factors are needed, they may be cost effective for other applications.
Cross-flow and counter-flow LAMEE devices have been constructed and tested in laboratory RAMEE system loops. The laboratory test prototypes for LAMEE devices have not performed as expected. In particular, the test systems have not reached steady-state operating conditions during a reasonable test period. Moreover, the internal geometry of the air and liquid flow channels are known to be far from the simple geometric configurations with uniform, equally distributed mass flow conditions assumed in the reported theoretical models.
Several key problems exist with the past research and development efforts for LAMEE devices. First, simple theoretical models of RAMEE or HVAC systems containing LAMEE devices, with overly simplified internal geometries and physics, fail to model what is physically occurring within the system. For example, each fluid flow will self adjust in a few seconds to distribute its local mass flux to minimize the pressure drop across the exchanger as a whole unit for each type of fluid, flow channel geometry, Reynolds number, Rayleigh number, and total mass flow rate. Within a fluid, both viscous flow forces and buoyancy forces can alter the flow streamlines. For example, buoyancy forces, caused by fluid density gradients, may result in unstable mal-distributed flow when the fluid density increases with height (i.e. counter to gravity) and the viscous forces are not sufficient to cause a uniform flow and so avoid a mal-distribution of flow within an exchanger. With some flow configurations in an exchanger, such flow conditions are likely to occur for laminar liquid flows but not the air flows. The enhanced performance of stable flows with enhancing buoyancy effects that self correct mal-distributions of flow are not exploited in existing systems.
When the self-adjusted flow is steady, the rate of entropy generation due to viscous (laminar or turbulent) flow will be a minimum for each flow channel and collectively for all the channels for each fluid (air or liquid desiccant) in the LAMEE. Due to small geometric variations and destabilizing buoyancy effects in each channel and among all the flow channels for each fluid, the self-adjusted flow distribution will not, in general, be such that the fluid mass flux is equally distributed among all channels or is uniformly distributed in each channel for heat and mass transfer through the semi-permeable membrane surfaces in a LAMEE. In order to minimize the declination of performance of each LAMEE due to the non-uniformities of flow distribution, the design specifications must be very complete for each and all independent performance influencing factors. When the uneven flow distribution leads to unequal flows among channels and/or poor non-uniform area integrated or locally averaged heat and water vapor transfer rates, the flow is mal-distributed in the exchanger for energy exchange. Mal-distribution of flows in any LAMEE in a RAMEE system will cause the performance of the system to be sub-optimal. Mal-distribution of flow will be especially prevalent for laminar flows with destabilizing buoyancy effects within each liquid channel and among the many liquid flow channels of a LAMEE. However, mal-distribution can also occur with transition and turbulent flows. Local flow instabilities, due to channel flow surface geometry when the flow is above threshold Reynolds numbers, will induce local turbulent mixing that can reduce mal-distributed flow in each channel and will increase both the pressure drop and convection coefficients. Exploiting fluid flow turbulence instabilities for enhanced convection coefficients and reduction of flow mal-distribution in exchangers has not been fully recognized or exploited in HVAC exchanger designs.
Further, LAMEE devices constructed with very flexible membranes need more detailed design and construction specifications for each local flow region in flow channels than more rigid flat-plate heat exchangers if they are to exceed the performance factors required for buildings {i.e. ASHRAE Std. 90.1 and 189.1} when tested using an accepted international standard {i.e. ASHRAE Std. 84} and/or approach the theoretical performance factors put forward by modelers. There is no indication that previous researchers and inventors have fully understood the complexities of the physical problems or were aware of the large number of independent design factors that influence the performance of the exchangers.
The key problems with existing RAMEE type energy recovery systems and HVAC systems having one or more LAMEE type devices for air conditioning supply air for buildings are closely related to the research and development problems set forth above. Typically, the factors that impact on the performance are not considered as a complete set if they are considered at all.
The steady-state performance of a passive RAMEE system is not characterized by a single factor as are some simple systems (e.g. pumps and motors). Rather, the performance may be characterized by a set of six dimensionless performance factors (i.e. four system effectiveness values for the measured fraction of the maximum possible steady-state sensible and latent energy transfer under summer and winter standard test conditions and two RER values for the measured fraction of auxiliary energy used with respect to the total energy transferred between the supply and exhaust air streams for the summer and winter test conditions). The set of performance factors, Pf, can be referred to as the dependent objective dimensionless ratios determined by analyzing the data from two standard steady-state tests for a passive RAMEE system.
The set of dimensionless ratios or factors that cause changes to the values in Pf are independent factors, If, because each one, or collectively several or all, will, if changed significantly, change one or more of the factors in the set, Pf. Mathematically, the relationship is expressed such that the dependent dimensionless set Pf is only a function of a predetermined dimensionless set, If, the operating conditions for the inlet air temperature and humidity (i.e. one standard test condition for winter and another for summer), and the uncertainty in the measured test data for both Pf and If or in short Pf(If) and where the standard test conditions are constrained by steady-state or quasi-steady-state operating conditions for each test.
Existing LAMEE devices and passive RAMEE systems have not been designed to meet specified performance factors other than designing the LAMEE device with an internal geometry similar to flat plate heat exchangers constructed using stiff elastic solids. That is, the systems have not met the desired set Pf because not all the factors in the set If were understood, considered, measured or specified.
A need remains to specify or predetermine a complete set of design parameters to construct a LAMEE and, for any inlet air conditions, select a narrow range of system operating conditions (i.e. the complete set If) if the RAMEE systems using two identical LAMEEs are to exceed all the required performance factors in the set Pf. When the design specifications are complete, the set Pf for a passive RAMEE and its two LAMEEs will be predictable in design, reproducible in manufacturing, and with reproducible and certifiable steady-state standard test results. Another need remains for LAMEEs used in a passive RAMEE system having an increased effectiveness. The LAMEEs need to be designed and operated to satisfy conditions that are typical for conventional energy exchange systems and that are required through international standards or local or state building codes.